Of course it's OK to be an introvert!
And of course introverts are adapted to life... it just depends on the kind of life you want to adapt to.
Aren't the USA in general pretty extraverted? I've read that extraverted behaviour (whatever that is) is seen as "normal"/"useful" etc., while the introverts are an embattled minority who have to defend themselves constantly. ("No, I don't hate people. No, I'm not lethargic. No, I... oh, just shut up.") So that is true?
Where I'm from it's the other way round. My country is much more introverted than the States, and the area my family is from is still more introverted. So it's "no, I'm not shallow. Nor unable to concentrate either, thank you very much. Yes, I love to go out and meet people. No, that's not because I'm unable to let myself be enough for me. No, I've got nothing against quiet evenings by the fireside. But this is the seventeenth quiet evening in a row. I want a loud evening!" and so on.
Introverts are simply able to make the most of the experiences they have. They're sensitive and perceptive. They don't need to rush around from one new experience to the next, because they can fully appreciate the kind of experiences they've got. They're focussed. Some quotes?
When we dare to be really quiet we can come very close to our selves and the worlds innermost being. - James Carroll
Without going outside, you may know the whole world. - Lao Tzu
There is a hell - to be alone. And a heaven - to be able to be it.
- Paul Bjerre